The present invention relates to the manufacture of integrated circuits on a substrate. More particularly, the invention relates to a method and apparatus for reducing backside contamination of substrates during processing.
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin layer on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to generally as chemical-vapor deposition (xe2x80x9cCVDxe2x80x9d). Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired layer. Plasma-enhanced CVD (xe2x80x9cPECVDxe2x80x9d) techniques, on the other hand, promote excitation and/or dissociation of the reactant gases by the application of radio-frequency (xe2x80x9cRFxe2x80x9d) energy to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the temperature required for such CVD processes as compared to conventional thermal CVD processes. These advantages are further exploited by high-density-plasma (xe2x80x9cHDPxe2x80x9d) CVD techniques, in which a dense plasma is formed at low vacuum pressures so that the plasma species are even more reactive. xe2x80x9cHigh-densityxe2x80x9d is understood in this context to mean having an ion density that is equal to or exceeds 1011 ions/cm3.
Because these processes are used in the precise manufacture of small-scale devices, it is especially desirable to limit the incidence of damage to the substrates during processing. Generally, silicon substrates used for processing are positioned onto a support, typically made of alumina, within a process chamber. The substrate is subject to expansion during processes that heat it with the plasma, which typically has a temperature of 400-800xc2x0 C. While the heat of the plasma also causes the alumina support to expand, there may be a considerable difference in the degree of expansion of the silicon substrate when compared with the alumina support. This is because alumina has a lower coefficient of thermal expansivity than does silicon (or most other semiconductors) and because the alumina surface coating is actively cooled to at or near 65xc2x0 C. The different expansions of the substrate and the support may result in scratching on the side of the substrate in contact with the support.
In some instances, the alumina support may be covered with a SiO2 layer. The temperature changes resulting from the plasma heating may similarly cause damage to that layer, such that some SiO2 flakes may adhere to the back side of the substrate. In further subsequent processing of the substrate, the flakes may fall off the substrate onto the front side of another substrate, thereby reducing overall device yield.
Embodiments of the invention are directed to a method for preparing a substrate that reduces the level of contamination of the back side of the substrate. The substrate is positioned within a chamber that has a substrate receiving portion, but in a location not on the substrate receiving portion. A gaseous flow is provided to the chamber, from which a plasma is struck to heat the substrate. After the substrate has been heated, it is moved to the substrate receiving portion for processing. In one embodiment, the plasma is a high-density plasma.
In certain embodiments, the substrate is positioned within the chamber by situating it on a plurality of lift pins, which may be electrically conductive. The substrate may then be moved to the substrate receiving portion when it is ready for processing by retracting the lift pins.
In some embodiments, the temperature of the substrate is monitored, with the substrate being moved to the substrate receiving portion when it reaches a predetermined temperature. This predetermined temperature may be the processing temperature at which the substrate is processed. The substrate temperature may be monitored by detecting infrared emission. In another embodiment, the substrate is moved to the substrate receiving portion after a predetermined time since striking the plasma has elapsed.
The methods of the present invention may be embodied in a computer-readable storage medium having a computer-readable program embodied therein for directing operation of a substrate processing system. Such a system may include a process chamber, a plasma generation system, a substrate holder, a gas delivery system, and a system controller. The computer-readable program includes instructions for operating the substrate processing system to form a thin film on a substrate disposed in the processing chamber in accordance with the embodiments described above.